A. Technical Field
The present invention relates to a LED illumination system, and more particularly, to systems, devices and methods of employing a LED driver control loop for smooth brightness adjustment and flicker suppression.
B. Background of the Invention
Semiconductor-based solid-state lighting (SSL), until recently associated mainly with simple indicator lamps in electronics and toys, has become as bright and more efficient than other lighting technologies. In particular, the enormous technology improvements have been achieved on light emitting diodes (LEDs) over the past years. LEDs have been available for various wavelengths, and suitable for white illumination. Lifetime of LEDs is also extended to more than 100 thousand hours, and can work at input powers up to many watts.
LEDs are connected in series as a LED string for use in lighting applications. Each power LED in the LED string used for illumination requires a nominal current anywhere in the range of 35-1400 mA, a forward voltage drop of 3V and large manufacturing tolerances. The LED strings are typically powered by switched mode power supplies, and linear regulators are normally used to stabilize the LED output current for stable brightness.
FIG. 1 illustrates a standard LED illumination system 100, and FIG. 2 illustrates exemplary time diagrams of signals in a LED illumination system 100. The system 100 comprises a transformer 106, a full-wave rectifier (FWR) 108, and a LED light module 110. The transformer is coupled to an AC power source 102 at any wall outlet, and converts a high-voltage AC supply voltage VSUP down to a low-voltage AC supply voltage VT. The FWR 108 is further coupled to the transformer 106, and rectifies the voltage VT to a voltage VAC. The voltage VAC functions as a switched mode power supply for the LED light module 110. The voltage VAC is further regulated in the LED light module 110 to provide a stable LED drive current for illumination.
The transformer 106 may be a magnetic transformer or an electrical transformer. The magnetic transformer is a conventional approach that requires bulky magnets. The AC supply voltage VSUP from the wall outlet is normally a sinusoidal signal having an amplitude of 110V or 220V at 60 Hz (curve 202). The magnetic transformer reduces the amplitude to a lower magnitude, e.g. 12V, while maintaining the same frequency (curve 204) for the resulting supply voltage VT. However, the electrical transformer can largely reduce the magnet size by first generating a high-frequency signal at several MHz. The amplitude of this high-frequency signal is modulated by the AC supply voltage VSUP at 60 Hz. The electrical transformer subsequently reduces the amplitudes of the high-frequency voltage, and thus that of the corresponding envelope voltage (curve 208). The envelope voltage is extracted for output as VT which has a reduced amplitude (e.g., 12V) at 60 Hz. Regardless of the transformer type, the FWR 108 subsequently generates a half-wave voltage VAC (curves 206 or 212) corresponding to the voltage VT generated by the magnetic or electrical transformer. Each half wave pulse of VT is associated with a powering or illumination cycle for a LED light in the module 110.
When an electrical transformer is involved, it is apparent that two consecutive powering cycles are separated by a dead band in the AC power supply VAC (curve 212). In the dead band, the supply VAC is zero and provides no illumination power. This dead band is generated mainly due to an internal oscillator employed in the electrical transformer, and this internal oscillator starts at a voltage Vosc varying among powering cycles (curve 210).
The LED illumination system 100 may further comprises a dimmer 104 which is used to control the brightness of the LED light. The dimmer is placed between the AC power source 102 and the transformer 106. The brightness of the LED light is modulated by disabling current injection for a short period of time tdim during each powering cycle, and in particular, the dimmer is used to reset the voltage VSUP to zero during this period and unavoidably results in a dead band between two consecutive powering cycles.
Variation of the dead band width is often associated with a visual artifact issue. The supply VAC remains zero in the dead band, and therefore, the dead band is directly associated with illumination energy loss for the corresponding powering cycle. When the dead band width varies significantly during consecutive powering cycles, brightness of the LED light varies and directly causes visual artifacts, most commonly perceived as flickering or blinking by human eyes. In particular, a harmonic artifact shows up as flickering at 120 Hz for the 60 Hz dead bands. This visual artifact issue exists in a LED illumination system 100 involving the electrical transformer 106 and/or the dimmer 104, since the dead band width is normally not sufficiently controlled in such a system.